Technical Field
The present invention relates to a telescopic boom extension device for extending and retracting a telescopic boom mounted on a mobile crane.
Related Art
Mobile cranes such as a rough-terrain crane, for example, generally include a multistage telescopic boom. The telescopic boom is extended and retracted using a hydraulic cylinder in general. In particular, devices for extending and retracting a telescopic boom using a single double-acting hydraulic cylinder have been proposed (hereinafter, referred to as “extension device”) (for example, refer to JP 7-267584 A, JP No. 4612144, and JP No. 4709415).
The extension device is structured as described below.
The multistage telescopic boom includes bottom-stage and top-stage booms so called the base boom and the top boom, respectively, and one or more booms placed between the foregoing booms, which are so called intermediate booms. When the telescopic boom includes a plurality of intermediate booms, the intermediate boom adjacent to the top boom is referred to as a first intermediate boom, and the other intermediate boom adjacent to the first intermediate boom is referred to as a second intermediate boom while the other intermediate boom adjacent to the second intermediate boom is referred to as a third intermediate boom, and so forth. Each of the booms extends (slides forth) and retracts (slides back) relative to the adjacent boom and is kept by a boom fixing pin (hereinafter, referred to as “B pin”) in the fully-retracted state and the fully-extended state. In the telescopic boom, the top boom is extended first, sequentially followed by the intermediate booms.
At the extension device, one end (cylinder rod-side end) of the single hydraulic cylinder is coupled to the base end of the base boom. When the booms are in the fully-retracted state, the adjacent booms are coupled together by the B pins. First, a cylinder tube of the hydraulic cylinder is coupled to the top boom. The two are coupled by a cylinder fixing pin (hereinafter, referred to as “C pin”), and the B pin is removed from between the top boom and the first intermediate boom to allow the top boom to slide relative to the first intermediate boom. When the hydraulic cylinder extends in this state, the top boom extends relative to the first intermediate boom.
When the top boom enters in the fully-extended state relative to the first intermediate boom, the top boom is coupled again to the first intermediate boom by the B pin. The C pin is removed from between the top boom and the hydraulic cylinder to retract the hydraulic cylinder. Then, the hydraulic cylinder is coupled to the first intermediate boom by the C pin, and the B pin is removed from between the first intermediate boom and the second intermediate boom to extend the hydraulic cylinder in this state. Accordingly, the second intermediate boom extends relative to the third intermediate boom. In this manner, each of the booms extends sequentially relative to the adjacent boom, and the entire telescopic boom is finally in the fully-extended state. In the reversed manner, the telescopic boom is retracted.
At a conventional extension device, the B pins and the C pins are driven by a hydraulic actuator. The hydraulic actuator is placed in the vicinity of the cylinder tube of the hydraulic cylinder in general. Accordingly, a pressure oil (operating oil) serving as a drive source for the hydraulic actuator is supplied from a hydraulic pressure source (hydraulic pump) via a hydraulic pipe. As described above, each of the booms slides relative to the adjacent boom, and the pipe for supplying the operating oil is generally a hydraulic pressure hose with a hose reel.